1. Field of the Invention
The invention relates to a liquid crystal display device and more particularly to a liquid crystal display device using ferroelectric liquid crystals.
2. Description of the Related Art
In recent years, display devices have played, as an information transmitting means, an important part in development of information apparatuses including microcomputers and the like. A typical display device is CRT (cathode ray tube), while liquid crystal display devices have been broadly used recently since they can be miniaturized in construction and can be reduced in power consumption. A liquid crystal display device of a larger screen and capacity is further strongly expected to be realized.
Given attention now are liquid crystal display devices using ferroelectric liquid crystals capable of exhibiting chiral smectic phases. The ferroelectric liquid crystals have a spiral construction in chiral smectic phase usually. When they are sealed in a thin cell, an interface effects to loosen the spiral construction, so that there coexist some domains wherein liquid crystal molecules 18 tilt at +.THETA. with respect to normal line 17 of the smectic layer and other domains wherein the liquid crystal molecules 18 tilt reversely at -.THETA. with respect to the normal line, as shown in FIG. 6A. Voltage is applied to the domains to cause the liquid crystal molecules to show uniform spontaneous polarization as oriented unidirectionally as shown in FIG. 6B. When voltage is reversed, the liquid crystal molecules are oriented uniformly in a reverse direction to that of FIG. 6B as shown in FIG. 6C. In the light of the fact that birefrigence changes following application of voltage in a normal and reverse direction as shown, a polarizing plate may be used to constitute a liquid crystal display device.
The liquid crystal display device can keep the initial molecular orientation as shown in FIG. 6D due to an orientation restraining force by the interfaces even after electric field is vanished, thereby exhibiting a high memorizing effect which is quite effective for a high duty multiplex drive display. For obtaining the above memorizing effect in the two states of the tilt angles +.THETA. and -.THETA., it is required to reduce difference of the orientation restraining force between upper and lower transparent substrates to diminish asymmetry of oriented liquid crystal molecules so as to allow coexistence of specific domains of the molecules tilted at +.THETA. and -.THETA., respectively.
A typical prior art of this kind of liquid crystal display device is as shown in FIG. 7, wherein a pair of transparent substrates 21 and 22 have on their opposite surfaces linear transparent electrodes 23 and 24 which are arranged as stripes forming a matrix electrode structure, insulating layers 25 and 26, and orientation films 27 and 28, respectively. Ferroelectric liquid crystals 29 are sealed between the transparent substrates 21 and 22, and polarizing plates 30 and 31 are laid on the outer surfaces of the substrates, thereby forming a ferroelectric liquid crystal display device 32.
The liquid crystal display device has diminished asymmetry in orientation of the liquid crystal molecules to obtain memorizing effect of the two states in which liquid crystal molecules tilt at +.THETA. and -.THETA..
The conventional liquid crystal display device is not applied with voltage at a region (non-pixel part) other than intersections (pixel part) of the extending directions of the linear transparent electrodes 23 and 24 and has diminished asymmetry in orientation of liquid crystals, so that it includes both of the foregoing two kinds of domains wherein the liquid crystal molecules tilt at +.THETA. and -.THETA., respectively. Hence, the non-pixel part 35 on the screen of the liquid crystal display, when light passes therethrough, has bright and dark (as hatched) regions to cause the screen to look rough, resulting in that displayed images cannot be given a uniform quality and are deteriorated in grade.
When the conventional liquid crystal display device is driven, voltage is applied from one ends of the linear transparent electrodes. If the linear transparent electrodes have large electrical resistance, voltage substantially decays at the other ends of the electrodes. As a result, voltage to be applied to liquid crystal layers is not uniform, leading to non-uniformity of display properties of the liquid crystal display device. The problem may be solved by such method as shown in Japanese Unexamined Patent Publication No. 63019/1990 that a conductive film as of metal having lower electrical resistance in comparison with the linear transparent electrodes is disposed lengthwise of and in contact with the electrodes to lower electrical resistance thereof. Further, ferroelectric liquid crystal device in the conventional example has such problem of inversion of the memory states in pixels. An electric field is applied to liquid crystal molecules to bring them into the state as shown in FIG. 6(d) followed by switching them to the state of FIG. 6(b). When the electric field is taken out, the liquid crystal molecules are not kept in the state of FIG. 6(b) but return to the state of FIG. 6(d). The phenomenon has been explained that it arises due to reverse electric field which is to be generated by spontaneous polarization of the ferroelectric liquid crystals [Yoshida, et al; 13th Liquid Crystal Forum Report, 2Z15(1987)]. A further study details that the inversion of memory states tends to arise at edges of the electrodes. Japanese Unexamined Patent Publication No. 179915/1989) A metal wiring has been reported to be effective for prevention of occurrence of the inversion of memory states. (Japanese Unexamined Patent Publications Nos. 179915/1989, 280724/1989 and 63019/1990) Metal or the like when arranged on the linear transparent electrodes narrows the transparent part thereof, so that substantial numerical aperture of the display is made smaller. Also, the metal or the like when disposed also at non-pixel part to prevent lowering of the numerical aperture is apt to contact with an adjacent linear transparent electrode, so that voltage is applied also to other linear transparent electrodes than a selected one, resulting in that quality of displayed image deteriorates.